The problem with running tests from IDEA is that IDEA does not use any Gradle task whatsoever

Solution: Tell IntelliJ to explicitly use the Grails test environment.

It is pretty easy configuration as shown below:

Please note that I added -Dgrails.env=test to my default configuration for JUnit tests. Add it to your default configuration, instead of in a single test configuration. If you do that, the next time you ask IntelliJ to run a test, it will create a new JUnit configuration based on your default configuration. That it is to say: your tests will run in the Grails test environment.